The invention relates to GPS receiver devices, and in particular to methods for compensating frequency drift errors of a crystal oscillator in a GPS receiver device.
Many types of sources for generating accurate frequency are available. The specific type of frequency source implemented within a particular application is determined according to the design constraints of the particular application. Atomic clocks exhibit extreme levels of frequency accuracy, however, their size, cost, and absence of tuning range greatly limit their actual application within an electronic system. Similarly, frequency sources could be realized utilizing the piezoelectric effect of quartz crystals. The small size and relative accuracy of quartz crystal based frequency sources make them popular for most consumer based electronic devices.
The application determines the required type and frequency accuracy of a frequency source. A receiver applied in Global Positioning System (GPS) applications requires a local oscillator (LO) with a high level of frequency accuracy in order to rapidly acquire and maintain synchronization with the signals provided on the GPS carrier frequencies transmitted from satellites. An overview of GPS helps to explain the requirement for frequency accuracy of a local oscillator in a GPS receiver. GPS is a popular technique for position determination. GPS locates position using geometric principles. A constellation of GPS satellites orbits the earth. A GPS receiver can determine its exact position by knowing the positions of the satellites and calculating the distance from the GPS receiver to each of the satellites. The GPS receiver calculates the distance from each satellite to the GPS receiver by determining the time it takes for a satellite signal transmitted by the satellite to reach the GPS receiver. Thus, the GPS receiver must quickly search and acquire the satellite signals utilizing a reference frequency generated by the local oscillator.
Typically, a GPS receiver requires a crystal oscillator to generate reference frequency. However, the frequency drift error of the crystal oscillator can be quite large and is highly dependent on the temperature due to the nature of piezoelectric crystals. FIG. 1 shows an exemplary S-curve representing the frequency drift error versus the temperature. In some GPS receivers, temperature-compensated crystal oscillators are thus used, but they increase the production cost; Some GPS receivers do not use temperature-compensated crystal oscillators and do compensate the frequency drift error by other methods or devices.
In U.S. Pat. No. 5,654,718, a memory is used to store the frequency drift error over a temperature range. The frequency drift error at a specific temperature point can be acquired by looking up a table, however, a large memory is required. In U.S. Pat. No. 6,509,870, a ninth-order polynomial equation is used to fit the S-curve. A large number of operations are required to calculate the coefficients of the ninth-order polynomial equation and the frequency drift error according the ninth-order polynomial equation.